Norrish Type I and II reactions

This chapter is divided up in the same manner as in previous years with sections dealing with Norrish Types I and II reactions, oxetane formation and miscellaneous reactions relating to carbonyl compounds and related species. 

He, H. Y., Fang, W.H., Phillips, D. L., Photochemistry of Butyrophenone Combined Complete active space Self consistent Field and Density Functional Theory Study of Norrish Type I and II Reactions, J. Phys. Chem. A 2004, 108, 5386 5392. 

Several recent reviews have explored PCET [1, 5, 23, 24, 29, 30] and specifically HAT [1,16, 17] from reaction chemistry and theoretical perspectives. This chapter will not exhaustively re-examine this material, but rather introduce a descriptive framework for the electron and proton that adequately depicts both the geometric and mechanistic complexities of PCET and its relation to HAT. Most examples will be restricted to systems in which kinetics have been measured and discussed within a PCET framework. Accordingly, more classical topics, such as radical organic photochemistry (e.g., Norrish Type I and II reactions) will not be considered. 

A new coarse grained molecular dynamics model was developed to study the role of thermal, mechanical and chemical reactions in the onset of the ablation process of PMMA [58]. In this model, the laser energy is absorbed in different ways, i.e. pure heating and Norrish type I and II reactions. Mechanical stresses and pressure are dominant for very short pulses in the stress confinement regime and can initiate... 

As in past years this chapter describes the photochemistry of those carbonyl compounds where the reaction type is dictated by the carbonyl function. Thus Norrish Type I and II reactions, rearrangements, and cycloadditions are dealt with in this chapter, but reductions and reactions of enones will be covered in later chapters in Part III. The shift in emphasis away from the study of simple carbonyl compounds which was pointed out in Volume 6 has been noticeable throughout this year. 

When the polymers are exposed to ultraviolet radiation, the activated ketone functionahties can fragment by two different mechanisms, known as Norrish types I and II. The degradation of polymers with the carbonyl functionahty in the backbone of the polymer results in chain cleavage by both mechanisms, but when the carbonyl is in the polymer side chain, only Norrish type II degradation produces main-chain scission (37,49). A Norrish type I reaction for backbone carbonyl functionahty is shown by equation 5, and a Norrish type II reaction for backbone carbonyl functionahty is equation 6. 

Norrish Type I fission of the side chain carbonyl group again at C-4. - Laser flash irradiation has been used as a aethod for the production of n-butylkotene from cyclohexanone. The chemistry of this ketene was studied in detail. The cyclohexanones (9a) undergo both Norrish Type I and II processes on irradiation. The fluorinated compounds (9b) showed a preference for Norrish Type II behaviour. Within the Norrish Type II biradical fluorine substitution leads to a preference for cyclization rather than cleavage. The Norrish Type I biradical afforded a ketene rather than an alkenal. A study of the photochemical reactivity of the diones (10) has shown that both Norrish Typo I and Type II reactivity can take place. The Typo I Type II product ratio is dependent upon ring size. Thus dione (10a) affords the Type II products (11) and (12) while dione (10c) yields the Norrish type I products (I3c-15c) and low yields of the Norrish Type II products (11) and (12). Compound (10b) is intermediate between these results affording a Type I Type II ratio of 0.3. A mechanistic study of the reactions was carried out. - ... 

Our results in Figure 6 show that during photooxidation of the polymers the ,/ -unsaturated carbonyl groups are converted into saturated ketonic/aldehydic groups that can themselves be converted subsequently to nonluminescent products by Norrish Type I and II processes, e.g., carboxylic acids (2). For the two light-sensitive polyolefins considered here, Reaction 1 is likely to be the more important, because the concentration of the species is very low. 

Tension on molecular chains may either inhibit or promote reaction rates, for example, poly(ethylene-co-carbon monoxide) photolysed by competing Norrish Type I and II processes (cf. section 3.1.5). 

Pitts and Osborne have compared the y-radiolyses and UV photolyses of low molecular weight methyl ketones, which undergo both Norrish Type I and II processes. They were unable to formulate a detailed mechanism for the y-radiolyses due to the multiphcity of excitation processes and the large number of possible excited species present. However, the gross pathway of reaction appears to be similar to that initiated by UV photons (i.e., predominantly Norrish Type 1 and II processes). From the product ratios, the authors estimated that carbonyl groups are 4 times more reactive than methylene groups when exposed to y-rays (Table 3.3). 

Although the Paterno-Buchi reaction is of high synthetic potential, its use in organic synthesis is still not far developed. In recent years some promising applications in the synthesis of natural products have been reported. The scarce application in synthesis may be due to the non-selective formation of isomeric products that can be difficult to separate—e.g. 6 and 7—as well as to the formation of products by competitive side-reactions such as Norrish type-I- and type-II fragmentations. 

Reactions are known where both Norrish Type I and Norrish Type II reactions compete, and the substituents on and nature of the substrate will determine which leads to the major product." ... 

Norrish Type I and Norrish Type II Reactions in the Isolated Molecule... 

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